This invention relates to a process for carrying out electroless plating.
Electroless plating on insulating materials such as glass and plastics is usually carried out by subjecting an insulating material to be plated to a conditioning treatment for improving an adsorbing power of a catalyst for electroless plating on the surface of insulating material, contacting the insulating material with a catalyst solution for electroless plating such as an aqueous solution containing palladium chloride and stannous chloride to adsorb the catalyst for electroless plating on the insulating material surface, subjecting the resulting insulating material to a treatment for accelerating the activity of catalyst for electroless plating, followed by dipping in an electroless plating solution.
Heretofore, the activation accelerating treatment has been conducted, for example, by dipping in a treating solution containing a mixed acid of sulfuric acid and hydrochloric acid, or such a mixed acid and tartaric acid (Japanese Patent Unexamined Publication No. 50-81927), or by dipping in an aqueous solution containing NaOH and ethylenediaminetetraacetic acid (Japanese Patent Unexamined Publication No. 51-8127).
In the production of printed wiring boards today, electroless plating is an important technique for forming through-holes which connect circuit patterns separated by an insulating material. As the insulating material which is required to have high reliability used in printed wiring boards, there are used insulating resins reinforced with inorganic oxide fibers such as glass fibers. In order to form through-hole circuit connection in such insulating materials, it is necessary to provide electroless plating sufficiently on both the insulating resin and the inorganic oxide fiber exposed on inner walls of drilled through-holes. If the deposition of electroless plating on either one of them is insufficient, there are formed so-called pin-holes which are non-metallized portions and are retained even if a metal layer in a through-hole is thickened by electric plating after the electroless plating. Thus, molten solder used for mounting electronic parts on the printed wiring board does not contact with the metal layer in the through-hole sufficiently, which results in making electrical connection between the electronic parts and the circuit insufficient.
According to the prior art processes of dipping in a treating solution containing a mixed acid of sulfuric acid and hydrochloric acid or a mixture of mixed acid and tartaric acid, or an aqueous solution containing NaOH and ethylenediaminetetraacetic acid for activation accelerating treatment, there was a problem in that activation acceleration for both the insulating resin and the inorganic oxide fiber was insufficient.
Further, in the production of printed wiring boards, it is necessary to conduct electroless plating not only on the two kinds of materials of insulating resin and inorganic oxide fiber, but also on a metal constituting circuits for electrical connection between circuits. But peeling of the metal by thermal shock easily takes place, so that it is necessary to give good adhesive strength between the metal constituting this circuit and the metal deposited by electroless plating. According to the known activation accelerating treatments, the adhesive strength between the copper foil and the electrolessly plated film is sometimes lowered.